Selective oxidation using gold

This critical review covers the recent development of the catalytic properties of gold in the selective oxidation of organic compounds, highlighting the exciting contribution to the art of catalysis. The unique, outstanding properties of nanometre-scale particles of gold, a biocompatible non-toxic metal, have allowed the development of a new generation of stable and selective catalysts for the conversion of many organic feedstocks to valuable chemicals. A critical discussion of the results of different research groups is presented along with attempts to correlate the catalytic properties with catalyst morphology in non-equivalent series of experiments.Particular emphasis has been given to the international efforts towards optimised synthesis of products of industrial appeal such as propylene oxide, vinyl acetate monomer, cyclohexanol/cyclohexanone, gluconic acid and glyceric acid (168 references).     

Update on selective oxidation using gold

This critical review aims to update the recent development in the selective oxidation of organic compounds by gold catalysis, highlighting the progress in the last three years. Following the impressive developments in the last decades, several protocols for catalytic oxidation are today available, which are based on the extraordinary properties of gold in terms of catalytic activity, selectivity, reusability and resistance to poisons. Beside many other applications, gold can be recommended for green processes dedicated to fine chemicals, pharmaceuticals and the food industry owing to its recognized bio-compatibility. The collected literature is focused on experiments concerning the oxidation of different chemical groups and could be of interest, in the wide area of organic chemistry, for improving previous processes or for exploring new catalytic pathways (174 references).     

Surface coverage and size effects on electrochemical oxidation of uniform gold nanoparticles

We investigated the electrooxidative dissolution of uniformly distributed Au nanoparticles (NPs) without an extra adhesion layer or capping agent. Diblock copolymer micelles were exploited to fabricate the arrays of Au NPs where not only diameter of the particles but also inter-particle spacing, and thus coverage were finely controlled. The peak potential for electrochemical oxidation shifted greater as a function of coverage of NPs than the size.     

Selective guanine oxidation by UVB-irradiation in telomeric DNA

The combination of the transient absorption measurement and DNA damage quantification by HPLC clearly demonstrated the preferential excitation of guanine and its decomposition in quadruplex DNA by UVB-irradiation.     

Ultrasensitive electrochemical biosensor for uranium using deoxyribozymes with amplification by gold nanoparticles

A novel highly sensitive and specific electrochemical biosensor for detecting uranium based on specific Deoxyribozymes and gold nanoparticles (AuNPs) is reported. In this work, AuNPs provide excellent electrochemical signal transduction and a large surface area for immobilising numerous Deoxyribozymes, so a low detection limit of 3.24 ng L^?1 uranium and a good linear relationship over the range 5.94–35.1 ng L^?1 (r = 0.994) were obtained. The proposed biosensor presents high specificity and selectivity for uranium and is not affected by other metal ions. Thus, the biosensor protocol offers good selectivity, rapid speed and operational convenience for detection uranium in liquid waste.     

Multiplex electrochemical immunoassay using gold nanoparticle probes and immunochromatographic strips

We describe a multiplex electrochemical immunoassay based on the use of gold nanoparticle (Au-NP) probes and immunochromatographic strips (ISs). The approach takes advantage of the speed and low cost of the conventional IS tests and the high sensitivities of the nanoparticle-based electrochemical immunoassays. Rabbit IgG (R-IgG) and human IgM (H-IgM) were used as model targets for the demonstration of the proof of concept. The Au-NPs based sandwich immunoreactions were performed on the IS, and the captured gold nanoparticle labels on the test zones were determined by highly sensitive stripping voltammetric measurement of the dissolved gold ions (III) with a carbon paste electrode. The detection limits are 1.0 and 1.5 ng ml⁻¹ with the linear range of 2.5–250 ng ml⁻¹ for quantitative detection of R-IgG and H-IgM, respectively. The total assay time is around 25 min. Such multiplex electrochemical immunoassay could be readily highly multiplexed to allow simultaneous parallel detection of numerous proteins and is expected to open new opportunities for protein diagnostics and biosecurity.     

TEMPO mediated oxidation of carbohydrates using electrochemical methods

The oxidation of some readily available carbohydrates was investigated in aqueous medium using a catalytic amount of 2,2,6,6-Tetramethyl-1-piperidinyl-oxy (TEMPO) as redox mediator. The regeneration of oxoammonium (active form) was electrochemically performed in alkaline medium at a glassy carbon anode and at controlled potential (0.5 V vs. AgCl/Ag). The primary alcohol groups of the carbohydrates without free aldehyde group, were selectively oxidized to the corresponding sodium uronate with high Faradaic yields. The presence of free aldehyde or hemiacetal groups leads to a decrease in selectivities towards uronic acids and increases the amount of by-products.     

A sensitive immunosensor using colloidal gold as electrochemical label

A sensitive immunosensor using colloidal gold as electrochemical label is described. In this method, the capture protein was first immobilized on a carbon paste electrode surface through passive adsorption to bind quantitatively with corresponding antigen and colloidal gold labeled antibody to perform a sandwich assay. To detect the amount of the colloidal gold captured on the electrode surface, the colloid was first oxidized electrochemically to produce AuCl₄⁻ ions which were adsorbed strongly on the electrode surface. Adsorptive voltammetry was then employed for the determination of the adsorbed AuCl₄⁻ ions. A linear relationship between reduction wave peak current and the antigen concentration (human IgG) from 10 to 500 ng/ml is obtained with a detection limit of 4.0 ng/ml.     

Electrochemical oxidation mechanism of guanine and adenine using a glassy carbon microelectrode.

The electrochemical oxidation mechanism of guanine and adenine was investigated using a glassy carbon microelectrode and cyclic and differential pulse voltammetry. It is pH-dependent and the electron transfer process occurs in consecutive steps with the formation of strongly adsorbed dimers on the electrode surface for both compounds.     

Highly sensitive and selective electrochemical detection of L-cysteine using nanoporous gold

Nanoporous gold (NPG) with uniform pore sizes and ligaments was prepared by using a simple dealloying method. NPG electrodes exhibit excellent electrocatalytic activity towards the oxidation of CySH and the mechanism for the electrochemical reaction of CySH on NPG has been discussed. Interestingly, if the operating potential is fixed at 0.65 V, a strong current is observed and interferences by tryptophan and tyrosine are avoided. The calibration plot is linear in the concentration range from 1 μM to 400 μM (R²?=?0.994), and the quantification limit is as low as 50 nM. The NPG-modified electrode has good reproducibility, high sensitivity and selectivity, can be used to sense CySH in aqueous solution.

Redox chemistry of guanine and 8-oxyguanine and a comparison of the peroxidase-catalyzed and electrochemical oxidation of 8-oxyguanine

The electrochemical oxidation of guanine and 8-oxyguanine has been studied over a wide pH range in aqueous solution. Guanine is initially oxidized in a     

Spectrophotometric determination of iodides by the products of fluorescein halogenation using electrochemical oxidation

The conditions providing the formation of iodine derivatives of fluorescein as analytical forms for the spectrophotometric determination of iodides have been studied. The conditions of the simultaneous electrochemical oxidation of iodides and bromides at a platinum anode have been discussed. The results are presented concerning the determination of simultaneously present iodine and iodide in model solutions, as well as iodide ions in potassium bromide.     

Destruction of butyraldehyde isomers using silver catalyzed electrochemical oxidation

The destruction of butyraldehyde isomers has been studied using a silver catalyzed electrochemical oxidation technique. The destruction rates of nand iso-butyraldehydes were measured as a function of the concentrations of butyraldehyde isomers, silver and nitric acid in aqueous solutions, anode potential and temperature. More than 99% of n-butyraldehyde in 3M nitric acid was destroyed within 50 minutes by electrochemical oxidation at the anode potential of 1.6V versus a reference saturated calomel electrode (SCE). The process was found to be applicable to the destruction of butyraldehyde isomers in nitric acid solutions.     

Recent progress in electrochemical oxidation of saccharides at gold and copper electrodes in alkaline solutions

This article reviews the progress made in the past 10 years, on electrochemical oxidation of saccharides in alkaline media for gold and copper electrodes. The mechanism and processes associated with the electrochemical oxidation of saccharides at native and surface coated electrodes continues to be of great interest. Despite the effort and various mechanisms proposed, still the need for an electrochemically active material that understands the complexity associated with saccharides continues to increase as their detection poses a challenge for bioanalytical chemistry and liquid chromatography.     

Catalysis by supported gold: correlation between catalytic activity for CO oxidation and oxidation states of gold

X-ray absorption near-edge spectra and temperature-programmed oxidation and reduction data demonstrate that Au(I) and Au(0) are both present in working MgO-supported gold catalysts for CO oxidation. EXAFS data indicate gold clusters with essentially the same average diameter (about 30 A) in each catalyst sample. Thus, the results provide no evidence of an effect of gold cluster size on the catalytic activity, but both the catalytic activity and the surface concentration of Au(I) were found to decrease with increasing CO partial pressure (as Au(0) was increasingly formed), demonstrating that the catalytic sites incorporate Au(I).     

Stereoselective synthesis of azasugars by electrochemical oxidation

A new method using electrochemical oxidation has been exploited for the stereoselective synthesis of 2,3,6-trihydroxylated 5S-piperidine derivatives. The electrochemical method was successively used for the conversion of N-protected piperidines to N-protected 1-methoxypiperidines and for the conversion of 1-methoxy-2,3-didehydropiperidine derivatives to 1,2,3-triacetoxypiperidine derivatives. The method provided a new synthetic route to 2S,3S,6-triacetoxy-5S-methylpiperidine and 2R,3R,6-triacetoxy-5S-methylpiperidine.     

Enhanced electrochemical evolution of oxygen by using nanoflowers made from a gold and iridium oxide composite

We report on the synthesis of a composite made from iridium oxide and gold that has a flower-like morphology. The ratio of iridium oxide to gold can be controlled by altering the concentrations of the metal precursors or the pH of the solution containing the reductant citrate. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and laser confocal micro-Raman spectroscopy were applied to characterize the structures of the nanoflowers, and a mechanism of their formation was deduced. The nanoflowers display an electrocatalytic activity in an oxygen evolution reaction (OER) that is significantly enhanced compared to bare iridium oxide nanoparticles. The highest turnover frequency for the OER of the new nanoflowers is 10.9?s^?1, which is almost one order of magnitude better than that of the respective nanoparticles. These attractive features are attributed to the high oxidation states of iridium in the nanoflowers which is caused by the transfer of electronic charge from metal oxides to gold, and also to the flower fractal structure which is thought to provide a much more accessible surface than suspensions of the respective nanoparticle.

Study of the electrochemical oxidation mechanism of formaldehyde on gold electrode in alkaline solution

The oxidation of formaldehyde in alkaline solution was studied by in situ rapid-scan time-resolved IR spectroelectrochemistry（RS-TR-FTIRS） method.In the potential range between -0.7 V and 0.2 V,the gem-diol anions were oxidized(according to the 2765 cm^-1 of-ν_H-O and 1034 cm^-1 ofν_co downward IR bands) and the formate ions appeared(according to the 1588,1357 cm^-1 of the asymmetric and symmetricν_oco and 1380 cm^-1 ofδ_C-H upward IR bands) in aqueous solution.It was also confirmed that gem-diol anion was oxidized(according to the 2026,1034 cm^-1 downward IR bands) to formate ions (according to the 1595,1357,1380 cm^-1 upward IR bands) and water(according to the 3427 cm^-1 ofν_{H- O} upward IR band) in heavy water solution.The results illustrated that formaldehyde formed gem-diol anion in alkaline solution and was absorbed on the electrode surface;then gem-diol anion was oxidized to formate ions and water.